Block copolymers consisting of vinyl aromatic hydrocarbons and conjugated dienes are commonly used as raw materials in heat-shrinkable films for shrink packaging. The physical properties required of shrink films are, for example, spontaneous shrinkage resistance which reduces the phenomenon of the films spontaneously shrinking when the films are stored in an ambient temperature environment that is not temperature-controlled. Patent Documents 1-3 describe that excellent spontaneous shrinkage resistance can be achieved by using block copolymers consisting of vinyl aromatic hydrocarbons and conjugated dienes, wherein the copolymers have specific polymer structures.
In recent years, there has been a demand for the heat-shrinkable films used in shrink packaging to be capable of handling packaged articles of various shapes and wrapping methods, as well as achieving a film appearance of good design, so in addition to the aforementioned spontaneous shrinkage resistance, the raw materials need to satisfy various physical properties such as low-temperature heat shrinkability, transparency, strength, rigidity and printability.
On the other hand, in order to reduce the burden on the environment, recent years have seen movements to make films thinner (hereinafter referred to as “thinning”) to decrease the amount of raw material used and reduce the generation of carbon dioxide, so there is a desire for development of a material emphasizing excellent rigidity and strength capable of withstanding actual use even with thinning.
One means of efficiently raising the rigidity is to mix a vinyl aromatic hydrocarbon polymer or a copolymer consisting of a vinyl aromatic hydrocarbon and an acrylic acid ester into a block copolymer consisting of a vinyl aromatic hydrocarbon and a conjugated diene, as described in Patent Documents 4-6.
When using a vinyl aromatic hydrocarbon polymer, if the compatibility with the block copolymer is insufficient, there is a risk of the transparency and strength decreasing depending on the molding conditions, as well as the glass transition temperature becoming too high. Additionally, when using a copolymer consisting of a vinyl aromatic hydrocarbon and an acrylic acid ester, if the kneading and dispersion in the block copolymer is insufficient, the strength can be reduced, and the resistance to alcohol may be low. Therefore, the development of a material not requiring mixture of these polymers, mainly comprising a block copolymer consisting of vinyl aromatic hydrocarbons and conjugated dienes excelling in rigidity and strength was desired.    Patent Document 1: JP 3543917 B    Patent Document 2: JP 3766820 B    Patent Document 3: JP 3659928 B    Patent Document 4: JP2003-96262 A    Patent Document 5: JP S59-221348 A    Patent Document 6: JP S61-123517 A